1. Field of the Invention
The present invention relates to a machine tool having a spindle that rotates about a spindle axis and that comprises a receptacle for toolholders, and a plurality of tool changers distributed around the spindle, the gripper arm of each tool changer carrying a toolholder that can be transferred, by means of the gripper arm on which it is mounted, between a magazine position located at a distance from the spindle above the receptacle, and a working position on the receptacle.
2. Related Prior Art
A machine tool of this kind is known from DE 38 29 105 A1.
In the case of the known machine tool a sleeve surrounding the spindle can be displaced relative to the longitudinal axis of the spindle. Mounted on the sleeve are a plurality of tool changers with gripper arms each of which terminated at its lower end by a gripper carrying a toolholder. By means of the gripper arms, and by displacing the sleeve, the toolholders can selectively be brought into a working position or transferred from the latter into the sleeve.
The gripper arms are part of a parallelogram suspension which ensures that the toolholders are held with their axes substantially in vertical direction during the swinging movement between the working position and a magazine position. In the magazine position, the gripper arms are pivoted vertically upward so that the toolholders occupy a position above the receptacle, in the area of the sleeve, with the tapers of the toolholders accommodated in cavities formed in a radial projection on the sleeve. The cavities serve as space to receive the toolholder and to protect them from metal chips that may fly around in the working space of the known machine tool. It is ensured in this way that during a machining operation no metal chips can get onto the toolholders that are to be used next, and may stick to them because of the drilling fluid attaching to them, which otherwise could result in problems when changing-in and clamping the respective toolholder.
Further, with the known machine tool each gripper arm is provided with a cover serving to close the cavity when the arm is pivoted into its working position and the toolholder, consequently, is removed from the cavity. This prevents any metal chips from entering the cavity associated to that toolholder while the latter is in its working position.
The design of the grippers of the known machine tools is such that the toolholders are rotatably seated in them so that the respective gripper can remain in engagement with the toolholder even when the latter is in its working position. The grippers, consequently, have quite considerable radial dimensions.
It is a disadvantage of the known machine tool, on the one hand, that the circumferential space requirement is relatively large, due to the before-mentioned gripper dimensions, so that the machine tool may have considerable lateral dimensions in the presence of a given number of toolholders.
On the other hand, it is a disadvantage that the tool changer must perform a relatively long stroke in order to ensure that during transfer from the working position to the magazine position the tools, being aligned with their axes in vertical direction, are moved a sufficient length in upward direction so that in their magazine position they will not interfere with the machining operations that are carried out with the tool mounted in the spindle. This relatively long stroke is achieved by the fact that the parallelogram suspension comprises two relatively long bars which, in the magazine position, extend substantially transversely to the spindle axis, while in the working position they extend substantially in parallel to the spindle axis. The longer these bars, the longer is the stroke that can be achieved by pivoting the bars by approximately 90.degree. during the tool-changing operation.
This design leads to the condition that the known machine tool not only has a considerable width, but also requires a considerable height in order to permit the tool change to take place without interfering with the work flow.